The process of Q-switching involves the introduction of loss, for example with the use of an absorber or shutter, in a laser oscillator sufficient to prevent lasing while energy is being transferred to a gain medium. When the loss is removed, an intense photon flux builds exponentially from spontaneous emission and the coupling of these photons through a partially reflecting resonator mirror leads to the generation of a Q-switched laser pulse. The first report of short intense pulses of laser light, called giant pulses at the time, was from an actively Q-switched ruby laser [McClung, F. J. and Hellwarth, R. W.: “Giant Optical Pulsations from Ruby,” Journal of Applied Physics 33 3, 828-829 (1962)]. Shortly thereafter, passive Q-switching using an organic dye as a saturable absorber was reported [P. P. Sorokin, J. J. Luzzi, J. R. Lenkard and G. D. Pettit: “Ruby Laser Q-Switching Elements using Phthalocyanine Molecules in Solution,” IBM J, vol. 8, pp. 182-184, April, 1964; P. Kafalas, J. I. Masters and E. M. E. Murray: “Photosensitive Liquid used as a Nondestructive Passive Q-Switch in a Ruby Laser,” J. Appl. Phys., 35(8), 2349 (1964); and B. H. Soffer: “Giant Pulse Laser Operation by a Passive, Reversibly Bleachable Absorber,” J. Appl. Phys., 35, 2551 (1964)].
Over the past forty years a variety of active and passive loss modulation techniques have been used to generate Q-switched laser pulses. These Q-switched laser pulses have been as short as several tens of picoseconds or as long as several hundred nanoseconds, with pulse energies that may range from a small fraction of a microjoule to a significant fraction of a Joule.
In recent years the advent of diode laser pumped solid-state lasers and solid-state saturable absorbers has generated renewed interest in passively Q-switched lasers, including microlasers. In passive Q-switching the laser resonator contains a gain medium and an absorbing medium, both may be saturable and therefore nonlinear in response. As the gain medium is pumped, it may both accumulate stored energy and emit photons. Over many round trips within the laser resonator the photon flux may see gain, fixed loss, and saturable loss in the absorber.
If the gain medium saturates before the absorber, the photon flux may build but the laser will not emit intense pulses. On the other hand if the photon flux builds up to a level that saturates or bleaches the absorber first, the resonator sees a dramatic reduction in intracavity loss and the laser Q-switches, thereby generating a short intense pulse of light.
A conventional passively Q-switched microlaser 100 is illustrated in FIG. 1. The passively Q-switched microlaser 100 includes a gain medium 101 bonded to a saturable absorber 102 enclosed by a pair of dielectric coatings, 105 and 106, that form a resonant cavity. The dielectric coating 105 on the gain medium transmits the pump light, provided by a light source 114, and is highly reflecting (the high reflector) at the microlaser wavelength. The dielectric coating 106 on the saturable absorber is partially reflecting (the output coupler) at the microlaser wavelength and provides the optical output 108 from the microlaser. The faces of the gain medium 101 and the saturable absorber 102 that form the interface 112 may be coated dielectrically so as to reflect unabsorbed pump light back through the gain medium and transmit light at the microlaser wavelength.